People park their cars in the church parking lot and open up their trunks and decorate inside of them. Also usually they dress up themselves as well. Then they hand out candy for the kids that come.
It's kinda of a way for the kids to get a feeling like they are going "door-to-door" but instead it's car-to-car." Instead of just being give a handful of candy, it just makes it a little more fun/interesting.

The ones we've been to also had games/events/food inside the church as well.

I started this at our church a few years ago and every year it is a huge success. We also have a chili cook off for the men. We serve hotdogs -- we have a cake walk with silly music and one for senior citizens we a song from there generation -- it is a ton of fun! Lots of pics on line. Good luck

and seems to be a safe alternative to the old fashion door to door.
It's a shame that kids today have to be so worried in their own neighborhoods. I grew up in the era of the razor blades in the apples scare in Michigan. I remember I was allowed to go to exactly two houses when I went trick or treating. It just wasn't safe.